Perua4

In both drama, the second lead is a writer whose identity is hidden from the public.
Main leads were suffered together abuse in the childhood and they have deep psychologic scars from that experience. In childhood ML protected the girl in both stories, ML in these shows were chaebols while FLs lived in low to middle class families.

Park Seo Joon is the 2L in KMHM and First lead in WWWSK. And in both shows there is a strong chemistry between the main leads .

WWWSK has a lighter vibe but they are both excellent dramas,

Both are focused in a girl with a fate related to demons and their love story with a cold and powerful immortal and have to face strong opposition from the "dark side" and from possessive and strong ladies loving MLs.

In SWAS , ML is not immortal but the reincarnation of a man cursed by his lover in a past life and they meet again falling in love and healing their karma. SWAS is a mini serie.

Secret Angel has the following points in common with Hi! School - Love On :

In both dramas, there is a female angel protecting a guy because of an accident and they eventually get in love. Their love is deep and pure leading to many cute moments. Moreover in these stories there is a jealous female opponent to the naive angel, the ML’s best friend who is secretly in love for him.

HSLO however is a youth romance, they are in high school. This is not the case for Secret Angel.